In an electric power steering apparatus (EPS) which has a motor control unit and applies a steering assist force to a steering system of a vehicle by a rotational force of a motor, the steering assist force is applied to a steering shaft or a rack shaft by a transmission mechanism such as gears and a belt with driving power of the motor via a reducer. Such the conventional electric power steering apparatus performs a feedback control of a motor current in order to precisely generate a torque of steering assist force. The feedback control is to adjust an applied voltage to the motor such that a difference between a steering assist command value (current commanded value) and a detected value of the motor current becomes small. Adjustment of the applied voltage to the motor is generally performed by adjusting a duty in a pulse width modulation (PWM) control.
Explaining a general configuration of an electric power steering apparatus with illustration in FIG. 1, a column shaft (steering shaft) 2 of a handle (steering wheel) 1 is connected to steered wheels 8L and 8R via reduction gears 3, universal joints 4a and 4b, a pinion and rack mechanism 5, tie rods 6a and 6b, and hub units 7a and 7b. A column shaft 2 is provided with a torque sensor 10 that detects a steering torque Ts of the handle 1. A motor 20 that assists the steering force of the handle (steering wheel) 1 is connected to the column shaft 2 via the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a steering assist command value of an assist (steering assist) command based on the steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by the vehicle speed sensor 12 and controls a current to be supplied to the motor 20 by a voltage control value Vref, which is the steering assist command value after compensation or other processing. Note that a steering angle sensor 14 is not a requirement and may not be disposed. The steering angle may be acquired by a rotational sensor connected to the motor 20.
The control unit 30 is connected with a controller area network (CAN) 40 that receives various information of the vehicle and the vehicle speed Vs can be received from the CAN 40. The control unit 30 may also be connected with a Non-CAN 41 that receives communication, an analog/digital signal, radio waves, or others that are different from those received by the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly includes a CPU (including an MPU, an MCU, etc.). Functions executed by a program inside the CPU are illustrated as an exemplary configuration as illustrated in FIG. 2.
Functions and operations of the control unit 30 are described with reference to FIG. 2. The steering torque Ts from the torque sensor 10 and the vehicle speed Vs from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Ts and the vehicle speed Vs using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after addition is limited of the maximum value thereof at a current limiting section 33. The current command value Irefm limited of the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtraction result I (=Irefm−Im) at the subtracting section 32B is proportional and integral (PI)-controlled at a PI-control section 35. The PI-controlled voltage control value Vref is inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback.
The compensating section 34 adds a self-aligning torque (SAT) 34-3 detected or estimated and an inertia compensation value 34-2 at an adding section 34-4. The addition result is further added with a convergence control value 34-1 at an adding section 34-5. The addition result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the characteristics.
In such an electric power steering apparatus, vehicles that have an automatic steering assist function (an automatic operation, a parking assist, etc.) and switches between the automatic steering control and the manual steering control have emerged in recent years. The vehicles having the automatic steering assist function perform the automatic steering control to set a target steering angle based on data from a camera (image), a distance sensor, or other apparatus and to cause an actual steering angle to follow the target steering angle.
In the automatic operation, environment surrounding the vehicle is recognized based on information from a radar, a camera, an ultrasonic sensor or the like and a steering angle command value that allows for safely guiding the vehicle is outputted. The electric power steering apparatus is capable of the automatic operation by performing a position control of the actual steering angle in such a manner as to follow the steering angle command value.
In the known electric power steering apparatus having the functions of the automatic steering control and the manual steering control in the related art, a back-in parking or a parallel parking is automatically performed by controlling an actuator (motor) based on relationship between a pre-stored traveling distance of the vehicle and a turning steering angle. That is, an automatic steering control apparatus recognizes a parking space from a positioning sensor such as an around-view monitor or an ultrasonic sensor and outputs a steering angle command value to the EPS-side. The EPS performs a position-control on the actual steering angle in such a manner as to follow the steering angle command value. As a result of this, the vehicle is guided into the parking space.
FIG. 3 is a diagram illustrating a control system of an electric power steering apparatus having the automatic steering control function. An automatic steering command unit 50 is inputted with various data from a camera and a positioning sensor (ultrasonic sensor or the like). A steering angle command value θtc for automatic steering is inputted into a position/speed control section 51 in an EPS-actuator function via a CAN or the like, and an automatic steering execution command is inputted into an automatic steering execution judging section 52 in the EPS-actuator function via the CAN or the like. The steering torque Ts is further inputted into the automatic steering execution judging section 52. An actual steering angle θr from the EPS-sensor is inputted into the position/speed control section 51 and a judgment result from the automatic steering execution judging section 52 is inputted into a torque command value gradual-change switching section 54. Further, the steering torque from the EPS-sensor is inputted into a torque control section 53 in an EPS-power assist function, and a steering assist torque command value from the torque control section 53 is inputted into the torque command value gradual-change switching section 54. A position/speed control torque command value Tp from the position/speed control section 51 is also inputted into the torque command value gradual-change switching section 54. According to the judgment result from the automatic steering execution judging section 52, the steering assist torque command value Tc and the position/speed control torque command value Tp are switched and output as a motor torque command value, thereby performing the drive-control of the motor via a current control system.
In this manner, a normal power assist is subjected to a torque control system. Meanwhile, the parking assist and the automatic operation are subjected to a position/speed control system of the steering angle or other parameters. There are problems such as that the control torque varies upon the switching between the torque control and the position/speed control, thereby making the switching over not smooth and that an unintentional self-steer occurs by a trigger due to variations in the torque upon the switching over.
To handle such problems, a conventional method to gradually change (gradual-change) the control torque in the torque control and the position/speed control is used in order to mitigate the torque variations. For example in Japanese Unexamined Patent Publication No. 2004-17881 A (Patent Document 1), when an automatic steering mode is released at a time point t0 as illustrated in FIG. 4, “Sθ=OFF” is reset and thereafter an angle control ratio μ is monotonously reduced within a predetermined time ΔT. This allows a command value of a current to be conducted in a motor not to drastically vary even upon switching between the control systems.